1. Field of the Invention
This invention relates generally to projection systems, and more particularly to a novel off axis projection system including a de-centered field lens group.
2. Description of the Background Art
Reflective liquid crystal displays (LCDs) provide many advantages over transmissive LCDs, and are, therefore, becoming increasingly more popular for use in projection systems. For example, transmissive displays typically have a limited aperture ratio (i.e., the total area available for light to shine through a pixel) and require pixel fill to separate the pixels, resulting in a pixelated image. The limitations of transmissive displays pose formidable problems in building bright, high-resolution displays at a reasonable cost. Reflective LCDs, on the other hand, include an array of highly reflective mirrors manufactured on a standard processed CMOS silicon chip back plane driver, using sub-micron metalization processes recently developed by VLSI process engineers, and do not, therefore, suffer from the limitations of the transmissive displays.
Although superior to transmissive displays in brightness and resolution, reflective displays do pose additional system design problems. For example, FIG. 1 shows a prior art, on-axis projector system 100 to include an illumination source 102, a polarizing beam splitter 104, a color separator 106, a plurality of liquid crystal displays (LCDs) 108(r, g, and b), and projection optics 110. Illumination source 102 generates a source beam of white light and directs the source beam toward polarizing beam splitter 104, which passes one portion of the source beam having a first polarity, and redirects another portion (an illumination beam) of the source beam having a second polarity along a system axis 112, toward color separator 106. Color separator 106 separates the illumination beam into its red, green, and blue components, and directs each of these colored illumination beams to a respective one of LCDs 108(r, g, and b). Each of LCDs 108(r, g, and b) is controlled by a system, e.g., a computer or other video signal source (not shown), and modulates the polarity of selective portions (i.e., pixels) of the colored illumination beams to form colored imaging beams, which are reflected back toward color separator 106. Color separator 106 recombines the colored imaging beams to form a composite imaging beam and directs the composite imaging beam back along the optical axis 112 of projection optics 110, toward polarizing beam splitter 104, which passes only the modulated portions of the composite imaging beam to projection optics 110. Projection optics 110 then focuses the modulated portions of the composite imaging beam onto a display surface (not shown).
Because the illumination beams and the imaging beams in system 100 both travel along the same path (i.e., axis 112), projection system 100 is considered an xe2x80x9con-axisxe2x80x9d system. On-axis projection systems generally require a polarizing beam splitter such as polarizing beam splitter 104, and, therefore, suffer from the following limitations. First, polarizing beam splitters are highly angular sensitive. Second, polarizing beam splitter 104 must perform both the polarizing function and the analyzing function, and must, therefore, work well for both orthogonal states (S and P) of polarization, thus requiring undesirable manufacturing compromises. Furthermore, polarizing beam splitter 104 introduces a significant path length through glass, which can induce undesirable aberrations in the incident and imaging beams, due to stress induced birefringence. Finally, polarizing beam splitters are very expensive, compared to, for example, polymer based polarizing films.
FIG. 2 shows an off-axis projection system 200 that does not require a polarizing beam splitter. Projection system 200 includes an illumination source 202, a condenser lens 204, a polarizer 206, a field lens 207, a reflective LCD 208, an analyzer 210, and a projection lens group 212. Illumination source 202 generates an illumination beam 214 that is focused by condenser lens 204 to pass through polarizer 206, and impinge on LCD 208 at a non-perpendicular angle (non-zero angle of incidence). LCD 208 modulates illumination beam 214 to form an imaging beam 216, and reflects imaging beam 216 toward projection lens group 212. Field lens 207 is disposed adjacent reflective LCD 208, and focuses the aperture stop (not shown) of illumination source 202 at a field stop (not shown) near the rear of projection lens group 212. The angular separation between illumination beam 214 and imaging beam 216 allows for the separation of polarizer 206 and analyzer 210.
Projection lens group 212 focuses imaging beam 216 to project a magnified image of LCD 208 on a display surface 220. In a configuration such as system 200, with a net average angle between LCD 208 and imaging beam 216, projection lens group 212 would typically be used as shown (i.e., not symmetrical about its optical axis 218) to avoid keystone distortion. Imaging beam 216 thus forms a non-zero angle with optical axis 218 of projection lens group 212.
The complexity of projection lens group 212 depends on the amount of angular separation between its optical axis 218 and the axis of imaging beam 216. In particular, for an angular separation between imaging beam 216 and optical axis 218 of projection lens group 212 adequate to permit a separate polarizer and analyzer (e.g., 12xc2x0), the total design field-of-view for the projection lens would be on the order of 30% larger than a similar on-axis system projecting a similar image on display surface 220. The resulting projection lens group 212 would tend to have excessive distortion, would be more complex, and would be more expensive than that required for the similar on-axis system. For typical distortion limits of  less than 0.25% in display applications, size benefits on the order of 30% reduction in track length can be achieved if the field-of-view is reduced.
What is needed, therefore, is a projection system, which allows the angular separation of the illumination beam and the imaging beam, without displacing and/or distorting the projected image, and without increasing the required field-of-view of the projection lens.
The present invention overcomes the problems associated with the prior art by providing a novel system and method for using off axis illumination in a reflective projection system. The invention facilitates the angular separation of an illumination beam and an imaging beam, without displacing and/or distorting the projected image, and easing the design requirements for a projection lens group.
The projection system includes an illumination source for emitting an illumination beam, a reflective display device for modulating the illumination beam to form a reflected imaging beam, a projection lens group, and a field lens group. The field lens group is de-centered with respect to the optical axis of the projection lens group and is disposed to bend the illumination beam and the imaging beam. The field lens group redirects the illumination beam to illuminate the display device at a non-zero angle of incidence, and redirects the reflected imaging beam along an optical path parallel to the optical path of the projection lens group. In a particular embodiment the display device is disposed on the optical axis of the projection lens group. In a more particular embodiment, the redirected portion of the optical path of the reflected imaging beam is coincident with the optical axis of the projection lens group. In another particular embodiment, the display device is tilted with respect to the optical axis of the projection lens group to accommodate the tilt in the focal plane of the projection lens group caused by the redirection of the imaging beam.
In one embodiment, the field lens group includes a centered field lens and an optical wedge. In an alternate embodiment, the field lens group includes a substantially de-centered field lens. In a particular alternate embodiment, the field lens group consists of a single, substantially de-centered field lens.
Another particular embodiment, where the display device is a liquid crystal display (LCD), further includes a linear polarizer disposed in the illumination beam and an analyzer (also a linear polarizer) disposed in the imaging beam.
Yet another particular embodiment, capable of multicolored projection, includes a color separator to separate the illumination beam into a plurality of colored illumination beams, a plurality of display devices, a plurality of field lens groups, and a color combiner. Each of the display devices modulates an associated one of the colored illumination beams to form an associated colored imaging beam. Each of the field lens groups is de-centered with respect to the optical axis of the projection lens group, and is disposed to redirect an associated one of the colored imaging beams along an optical path parallel to the optical axis of the projection lens group. The color combiner recombines the colored imaging beams to form the imaging beam directed to the projection lens group.
A method of projecting a display image is also described. The method includes the steps of providing a reflective display device, providing a projection lens group, directing an illumination beam to illuminate the display device at a non-zero angle of incidence, modulating the illumination beam with the display device to form a reflected imaging beam, and redirecting the reflected imaging beam along an optical path parallel to the optical axis of the projection lens group. In a particular method, the step of redirecting the reflected imaging beam along an optical path parallel to the optical axis of the projection lens group includes redirecting the reflected imaging beam along an optical path coincident with the optical axis of the projection lens group.